Method and system for improving alignment precision of parts in MEMS

ABSTRACT

An improved alignment precision of Micro-Electromechanical Systems (MEMS). A method includes two parts of MEMS separated by at least one rolling element having a first diameter, where the rolling element is maintained on one of the two parts using a thermally dissipative material, horizontally aligning the two parts by pivoting one of the two parts about the rolling element, and locking the two parts together.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. application Ser.No. 11/571,811, filed on Jan. 22, 2009, the contents of which areincorporated by reference in their entirety herein.

FIELD OF THE INVENTION

The present invention relates generally to fabrication techniques andMicro-ElectroMechanical Systems (MEMS) and more specifically to a methodand systems for improving alignment precision of parts in MEMS duringmanufacturing.

BACKGROUND OF THE INVENTION

Micron-sized mechanical structures cofabricated with electrical devicesor circuitry using conventional Integrated Circuit (IC) methodologiesare called micro-electromechanical systems or MEMS. There has been agreat deal of recent interest in the development of MEMS devices forapplications such as projection devices, displays, sensors and datastorage devices. For example, one of IBM's projects concerning datastorage device demonstrates a data density of a trillion bits per squareinch, 20 times higher than the densest magnetic storage currentlyavailable. The device uses thousands of nanometer-sharp tips to punchindentations representing individual bits into a thin plastic film. Theresult is akin to a nanotech version of the data processing ‘punch card’developed more than 110 years ago, but with two crucial differences: theused technology is re-writeable (meaning it can be used over and overagain), and may be able to store more than 3 billion bits of data in thespace occupied by just one hole in a standard punch card.

The core of the device is a two-dimensional array of v-shaped siliconcantilevers that are 0.5 micrometers thick and 70 micrometers long. Atthe end of each cantilever is a downward-pointing tip less than 2micrometers long. The current experimental setup contains a 3 mm by 3 mmarray of 1,024 (32×32) cantilevers, which are created by silicon surfacemicro-machining. A sophisticated design ensures accurate leveling of thetip array with respect to the storage medium and dampens vibrations andexternal impulses. Time-multiplexed electronics, similar to that used inDRAM chips, address each tip individually for parallel operation.Electromagnetic actuation precisely moves the storage medium beneath thearray in both the x- and y-directions, enabling each tip to read andwrite within its own storage field of 100 micrometers on a side. Theshort distances to be covered help ensure low power consumption.

FIG. 1 is a partial cross section view of such device (100). As shown,each cantilever 115 is mounted on a substrate 105 surmounted by a CMOSdevice 110, with a control structure 120, and comprises adownward-pointing tip 125 that is adapted to read or write (R/W) a biton the surface of the storage scanner table 130. Thanks toelectromagnetic actuator 135 storage scanner table 130 can move in atleast one dimension as illustrated by arrows. The part comprising thestorage scanner table 130, the actuator 135 and the support structure140 must be precisely aligned on the CMOS device 110, at a predetermineddistance. CMOS device 110 has all the required electronics to controlrequired functions such as R/W operations. In this implementationexample, alignment functional targets in X and Y axis are on the orderof ±10 μm (micrometer), while the functional gap between the storagescanner table 150 and the CMOS device 110 that works also as asupporting plate for the R/W cantilevers has a maximum distance of 6 μmwith sub-micron tolerance.

The combination of electrical and mechanical features associated withthe required part alignment accuracy leads to the use of dedicatedmanufacturing tools that directly impacts device cost. In the highvolume production of this kind of product for the consumer market suchinvestments would become very high due to an intrinsic conflict betweenthroughput (capacity) and precision alignment requirements. Therefore,there is a need for a method and systems for aligning efficiently partsof the MEMS during manufacturing, without requiring dedicated andcomplex manufacturing tools.

SUMMARY OF THE INVENTION

Thus, it is a broad object of the invention to remedy the shortcomingsof the prior art as described here above.

It is another object of the invention to provide a method and systemsfor reducing the friction between MEMS parts when aligning them.

It is a further object of the invention to provide a method and systemsfor improving self-alignment precision of MEMS parts duringmanufacturing while controlling the distance between these parts.

It is still a further object of the invention to provide a method andsystems for improving self-alignment precision of MEMS parts duringmanufacturing, according to rotational misalignment, while controllingthe distance between these parts.

The accomplishment of these and other related objects is achieved by amethod for improving alignment precision of at least two parts of anelectronic device, said at least two parts being in contact through atleast one rolling element, said method comprising the steps of,

-   -   aligning said at least two parts; and,    -   locking said at least two parts together.

Further advantages of the present invention will become apparent to theones skilled in the art upon examination of the drawings and detaileddescription. It is intended that any additional advantages beincorporated herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial cross section view of a device wherein the inventioncan be efficiently implemented.

FIG. 2, comprising FIGS. 2 a, 2 b, and 2 c, illustrates the mechanism ofthe invention for reducing the friction between MEMS parts to bealigned, for improving alignment precision.

FIG. 3 depicts how the spheres or bearings of FIG. 2 are temporarilyblocked.

FIG. 4, comprising FIGS. 4 a and 4 b, illustrates the concept ofsolder-reflow alignment process that can be used in conjunction with thefriction reduction mechanism of FIG. 2.

FIG. 5 is a partial cross section view of the device of FIG. 1 whereinthe invention is implemented.

FIG. 6, comprising FIGS. 6 a, 6 b, 6 c, and 6 d, illustrates the mainsteps of a first embodiment for the self-alignment of MEMS parts.

FIG. 7 illustrates a partial plan view of a device wherein the inventionis implemented and shows the dominant force vectors based on each paddesign, according to the first embodiment.

FIGS. 8 and 9 show the shapes of the pads used for aligning MEMS partsaccording to the first embodiment.

FIG. 10, comprising FIGS. 10 a, 10 b, 10 c and 10 d, shows an example ofthe control of the pad sizes and the alloy volumes.

FIG. 11 depicts a second embodiment for self-aligning MEMS parts, usinggravitational effect.

FIG. 12 illustrates an apparatus for applying a Z force to increase thegravitational effect.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

According to the invention there is provided a design strategy allowingstacking two or more parts of Micro-ElectroMechanical Systems (MEMS)with very high precise position via either a solder-reflow process,which could also form a final electrical and/or mechanical connectionbetween the parts of the MEMS, a gravitational effect based process, ora combination of both. Furthermore the invention offers a selfcontrolled correction of rotational placement errors and a self forced Zcontrolled height or functional standoff.

For sake of illustration, the description of the invention is based uponthe example given above by reference to FIG. 1 concerning a data storagedevice. Such data storage device is made of a MEMS with a moving table,also referred to as a scanner or scanner table, and relatedelectromagnetic controls, a CMOS device that has all the requiredelectronics to control read and write (R/W) function performance andcarrying a great number of single structures that are the R/W tips.

As mentioned above, there are precise functional requirements for thepart stack-up. Alignment functional targets in X and Y axis are in theorder of ±10 μm (micrometer), while the functional gap between thescanner table and the CMOS device that works also as a supporting platefor the R/W cantilevers has a maximum distance of 6 μm with sub-microntolerance.

According to the invention, the alignment system is based upon the useof rolling spheres or bearings to reduce friction between the movingparts of the MEMS. Preferably, the spheres or bearings are placed intocavities or niches that are present on one of the stacking elements.Such cavities can be, for example, etched on the surface of MEMS part.It is to be noticed that cylindrical bearings rotating along their mainaxis offer to move parts along the orthogonal direction only whilespheres rotating along their center offer to move parts along x and ydirections. It is also to be noticed that the spheres and bearings areused as spacers to provide a precise control of the distance betweenMEMS parts.

FIG. 2, comprising FIGS. 2 a, 2 b, and 2 c, illustrates the abovementioned spheres and bearings. FIG. 2 a is a partial cross section viewof two MEMS parts e.g., a CMOS device 110′ and a structure140′supporting a storage scanner table, similar to the ones shown onFIG. 1. CMOS device 110′ comprises a cavity 200 wherein a sphere 205 canrotate so that CMOS device 110′ and structure 140′ supporting a storagescanner table can be aligned with a low friction coefficient.Preferably, the system comprises three spheres (or two spheres and onebearing, one sphere and two bearings, or three bearings) that are notaligned so as to create three interface points between MEMS parts toalign. FIG. 2 b shows the preferred shape of cavity 200 consisting in acylinder that axis is perpendicular to the plan formed by the interfacepoints between MEMS parts to align. FIG. 2 c illustrates the use of abearing 210 and the preferred shape of the corresponding cavity 215. Asshown, the preferred shape of the corresponding cavity 215 is arectangular parallelepiped.

The rolling spheres or bearings that can be made of Tungsten carbide,offer a low friction coefficient to the moving parts that have relativemovement in respect to each others. Cavities hosting the spheres orbearings are made of a size such to encompass mainly the positioningtolerances of the stacked parts of the MEMS plus all the requiredmanufacturing tolerances. For example, the sphere can have a diameter of7 um, the cavities having a depth of 1 um.

Still preferably, the spheres or bearings are maintained and centered inthe cavities before positioning the MEMS parts. As illustrated on FIG.3, rotational elements are kept in their position by a temperaturedissipative material 300, that frees the rotational elements when othermechanisms are supposed to take over the relative positioning of theMEMS parts. Preferably, the temperature dissipative material consists ofa gel comprising water and glycerol. This gel fully dissipates duringthe temperature excursion in the front end portion of a soldering reflowprofile, between approximately 80° C. and 130° C. and that typically hasto last at least 140 seconds. This gel evaporates and leaves no residuesthat keep the spheres from rolling when the two parts of the stackedMEMS start to move. The gel may be obtained by mixing 10 g of glyceroland 6 g of water, stirred at a temperature greater than 45° C. in avacuum chamber until the gel is formed by volume reduction and loss ofwater. Vacuum mixing increases the water evaporation rate at lowertemperature and avoid the trapping of air bubbles within the gelformation during the mixing operation. Gel can be dispensed by a kind ofshower head placing a dot of gel into the cavities, this shower head canactively dispenses the gel e.g., by injecting the gel through needles,or transfer the gel in a passive way. Another solution consists indipping the spheres or bearings into the gel before placing them intocavities.

Once the alignment has taken place the locking mechanisms of the stackedMEMS, such as soldering, hold the spheres or bearings in place byfriction.

As mentioned above, the self-alignment of MEMS parts, using low costindustrial processes, can result from a specific soldering process, froma particular shape of the floor of the rotational element cavities, orfrom a combination of both.

The implementation of specific design of metal pads and the utilizationof selected soldering alloys such as standard eutectic Tin/Lead(63Sn/37Pb) or non eutectic Sn/Pb binary alloys such as Sn60/40Pb or5Sn/95Pb, 10Sn/90Pb, 3Sn/97Pb or other “Lead-free” alloys such asTin/Silver/Copper ternary alloys or other alloys that can be based onIndium or Silver, or Tin or other metals alloys allows taking advantageof the surface tension physics of the melted alloy deposit. Solder alloycan be selected based on the solder hierarchy required in the overallproduct manufacturing system and based on maximum acceptable temperatureexcursions that the different MEMS components can withstand to. Thewetting phenomenon between the metal pads and the alloy in liquid phasedrives the self centering operation along the X and Y axis between thetwo parts of the MEMS as illustrated on figure that shows two parts(400, 405) of the MEMS, each comprising a pad (410, 415) in contact withalloy (420), at the beginning (a) and at the end (b) of the reflowprocess.

These tension effects in molten solder can also be used to create, whenrequired in the same design, a complex system of forces for rotationalself-alignment (theta axis) by creating moments to pivot around variousfeatures. Additionally, by adjusting the relative sizes of pads andamount of alloy, the pressure exerted on the rotation element can becontrolled.

FIG. 5 illustrates a device 500, wherein one embodiment of the inventionis implemented, comprising two metal pads 505 and 510, linked bysoldering alloy 515. As mentioned above, the distance between the MEMSparts e.g., between CMOS device 110′ and the part comprising the storagescanner table 130′, the electromagnetic actuator 135′, and the structure140′, is determined according to the diameter of the rotational element520 and the depth of the corresponding cavity 525.

According to the device shown on FIG. 5, the X, Y, and theta alignmentscan be performed, along with the Z collapse against rotational elements,in a single reflow step.

FIG. 6, comprising FIGS. 6 a, 6 b, 6 c, and 6 d, illustrates the mainsteps of the alignment of MEMS parts. As shown on FIG. 6 a, thestructure 140′ supporting the storage scanner table comprises a pad 510and the CMOS device 110′ comprises a pad 505 and a cavity 525 wherein asphere is maintained and centered by a temperature dissipative material300 such as a gel comprising water and glycerol. A predefined amount ofsoldering material 515 is deposited on pad 505. When CMOS device 110′and structure 140′ supporting the storage scanner table are aligned,pads 505 and 510 are also aligned.

As described above, the temperature dissipative material 300 starts todissipate during the first step of the soldering reflow process andcontinues up to complete dissipation while the temperature reaches themelting point of the soldering material 515. The soldering materialchanges states and starts to wet the pad 510 as depicted on FIG. 6 b.The MEMS parts collapse one on top of the other until they hit thesphere 520, as shown on FIG. 6 c, that is free to accommodate a movementof the MEMS parts that are self-aligning as described by reference toFIG. 4. At the end of the self-alignment process, the pads 505 and 510are aligned and the temperature is decreased, locking MEMS partstogether, as depicted on FIG. 6 d.

Each pad design is inspired by the location and the resultingcontribution of the same to the resulting forces that will drive theself alignment of the stacked MEMS parts. Preferably, each MEMS part tobe aligned comprises at least three pads, at least a portion of each padof a part being exactly aligned to one pad of the other part when partsare precisely aligned. In a preferred embodiment, there are three padsforming a triangle i.e., defining a plane, two of them being longrectangular pads, these have demonstrated to have a stronger pullingforce along the direction orthogonal to the long side. These rectangularpads, being disposed according to an angle of approximately 90°, areresponsible to give a consistent contribution to the X and Ymacro-alignment but are responsible to achieve a precise micro-alignment(sub-micron level) of the metal pads and then of the MEMS parts. Makingthe pads rectangular and with a high aspect ratio between the two sidesis also satisfying one of the requirements for the collapsing feature ofthe Z control process.

The third pad has to maintain the same X and Y recovering action(forces) but it can be at a lower level when it is basically centeredbut has the option of becoming a strong contributor to the selfcentering forces when the misplacement is at macro-level (tens ofmicrons). The other main function of the latter pad design is to act asa pivotal point and to allow slight rotation of the system inassociation with the acting forces driven by the other two rectangularpads.

The definition of the design characteristics for the third pad resultedin a pad with a profile similar to a “Donut” where the resulting forcesact along the pad as if the pad itself would be a long rectangular pad,with a high ratio between the two different edges, something verysimilar to the two remaining pads.

The melted alloy will wet the mating pads creating the aligning forcesdriving a complete and low surface energy 3D structure that can bereached only when an exact overlap of the wettable surfaces (pads) ispresent.

FIG. 7 illustrated a partial view of a device wherein the invention isimplemented and shows the dominant forces vectors (arrows) based on eachpad design. As illustrated, a MEMS part 700 comprises pads 710, 715 and720 that are aligned to corresponding pads of another MEMS part 705,allowing the alignment of parts 700 and 705 during the solder-reflowprocess.

FIG. 8 depicts the corresponding pads of two MEMS parts i.e., a pair ofpads, as well as the dominant forces allowing their alignment. As it isunderstandable from this drawing, both pads should have approximatelythe same widths l and different lengths so as to determine a mainalignment direction. The greater misalignment distance that can becorrected is equal to approximately the half of the pad width i.e., l/2.

FIG. 9, comprising FIGS. 9 a, 9 b, and 9 c, illustrates an example ofpad design for the above discussed third pair of pads used as a pivotalpoint. Preferably, the internal radiuses R₁ and R₂ of the annular ringof both pads are equal, R₁=R₂, while the external radius of one of thepads is greater than the external radius of the second pad e.g., R₃>R₄.The greater misalignment distance that can be corrected is equal toapproximately the half of the difference between the external radiusesof both pads i.e., (R₃−R₄)/2.

Therefore, the use of two similar pair of rectangular pads, one pairbeing rotated of an angle approximately equal to 90° from the other, andof an annular pair of pads as discussed above, allows an X and Yalignment as well as a rotational adjustment.

The amount of soldering material used between corresponding pads forself-aligning MEMS parts and for soldering them must be preciselypredetermined. It must be great enough to ensure contact between padsand efficient tension effects for aligning MEMS parts but it must not betoo great so as to guarantee that spheres are in contacts with both MEMSparts.

The resulting combination of available volume of soldering alloy pairedwith the available wettable surfaces drives a distribution of the soldervolume achieving a 3D structure with the minimum surface energy.

The required amount of soldering material is reached by underestimationof the required volume, at equilibrium, for a specific height and padssurfaces. This will create an over consumption of the alloy with aresulting collapsing action that would tend to reduce the gap beyondwhat is imposed by the presence of the spheres.

FIG. 10 and the following tables show an example of the different paddimensioning based on the possible/available solder alloy volumes.Processes to deposit such small amount of soldering alloy do havedifferent costs and different tolerance to the targeted volume. Thetables were used to design targeted volumes with different surface areasbased on steps of fixed solder deposits.

FIG. 10 a illustrates a rectangular pad (1000) configuration aftersolder (1005) deposition and FIG. 10 b shows the rectangular pads (1000,1010) configuration after self-alignment operation and solder (1005′)consumption. The geometrical configuration is then computed with goodapproximation to a pyramidal frustum with parallel faces.

Assuming that,

-   -   b is the area of the pad (1000), on which alloy is deposited,        that width and length are both equal to 100 μm,    -   B is the area of the receiving pad (1010) that width is equal to        100 μm,    -   h is the height of alloy deposition, prior to joining and its        value is a variable of solder deposition process capability for        very small volumes. The value of h may be an independent        variable that drives the overall sizing of the pads geometry,    -   H is the targeted height of alloy between pads (1000, 1010),        and,

-   V is the alloy volume,    then,

$\begin{matrix}{V = {H \cdot \frac{B + b + \sqrt{B \cdot b}}{3}}} & (1)\end{matrix}$

and the length of the receiving pad (1010) is:

Height of alloy dep. h(μm) 15 14 13 12 10 Receiving pad length (μm) 560510 460 410 320

Likewise, FIG. 10 c illustrates an annular pad (1015) configurationafter solder (1020) deposition and FIG. 10 d shows the annular pads(1015, 1025) configuration after self-alignment operation and solder(1020′) consumption. The geometrical configuration is then computed withgood approximation to a conical frustum with parallel faces and acylindrical cavity of volume πR₁ ²H in the center.

Assuming that,

-   -   R₁ and R₂ are the radiuses of the empty circular areas in the        center of both annular pads (1015, 1025), R₁ and R₂ are both        equal to 50 μm,    -   R₄ is the external radius of the pad (1015), on which alloy is        deposited, it is equal to 150 μm,    -   R₃ is the external radius of the receiving pad (1025)    -   h is the height of alloy deposition, prior to joining and its        value is a variable of the capability of the solder deposition        process for depositing very small volumes. The value of h may be        an independent variable that drives the overall sizing of the        pad geometry,    -   H is the targeted height of alloy between pads (1015, 1025),        and,    -   V is the alloy volume,

then,

$\begin{matrix}{V = {H\;{\pi \cdot ( {\frac{R_{3}^{2} + {R_{3}R_{4}} + R_{4}^{2}}{3} - R_{1}^{2}} )}}} & (2)\end{matrix}$

and the external radius R₃ of the receiving pad (1025) is:

Height of alloy dep. h(μm) 15 14 13 12 10 Receiving pad radius (μm) 340325 310 290 260

In another embodiment the self-alignment process uses gravitationaleffect. According to this embodiment, there are two complementary etchedcavities on the two mating surfaces of the MEMS parts. As shown on FIG.11, wherein FIG. 11 a depicts a partial cross section view of MEMS parts110″ and 140″ and FIG. 11 b depicts the corresponding partialperspective view, the cavity 1105 formed in CMOS device 110″ looks likea flattened cone and the cavity 1110 formed in the structure 140″ is acone. Cavity 1105 comprises a flat floor wherein the sphere 1100 iscentered and maintain with a temperature dissipative material, such as agel, as mentioned above, and wherein the sphere can freely roll when thetemperature dissipative material is evaporated. The conical shape ofcavity 1110 allows the sphere to be self-aligned with the cone centerdue to gravitational effect, to reach a stable state. The cavities areformed such that MEMS parts are aligned when the sphere is aligned withthe cone center of cavity 1110. The distance A between the centers ofcavities 1105 and 1110 corresponds to the misplacement of the top MEMSpart while distance B, being equal to the radius of the flattened conesmaller face, is the maximum distance that can be recovered according tothe self-aligning mechanism.

FIG. 12 illustrates an apparatus for applying a Z force to increase thegravitational effect. Small, light, magnetic “weights” 1200 are placedon top of the upper MEMS part 1205, in a manner which is well centeredover the solder joints, or a subset of the solder joints. Magneticsolenoids 1210 with switchable current (and therefore switchable field)are located in a manner below the parts being joined, such that they arewell-centered under each magnetic weight. If these solenoids and weightshave well-behaved fields (and no other ferromagnetic structures arepresent which would distort the fields from the solenoids), then theforce on each magnetic weight is purely vertical (has no in-planecomponent) to within a given tolerance.

When the field in the solenoids is switched on, the magnetic weightsproduce a vertical force on the upper MEMS part 1205, driving the partagainst the sphere 1215 allowing the alignment of MEMS parts accordingto shapes of cavities. The amount of force is determined by the size andmagnetic permeability of the magnetic weights, and the design of andcurrent applied to the solenoids.

The magnetic weights are placed by robotics or other means prior to thestart of the reflow process. A single lightweight structure withmagnetic inclusions at the proper locations can simplify the placementof the magnetic components. Gravity should be sufficient in most casesto hold the magnetic weights in the proper locations. After cooling, themagnetic weights can be lifted off the bonded stack.

Placement of the magnetic masses may also be aided by providing groovesor other alignment feature in the top of the upper MEMS part. Ifcone-shaped, cylindrical, or square depressions are provided in theupper MEMS part, steel balls (which are widely available at low costwith precisely controlled dimensions) may be used as the magneticweights.

After the MEMS parts have been aligned a soldering process may beapplied to lock the parts together.

Even if the described example is based upon the use of glycerol basedgel for maintaining the rolling spheres/bearings before aligning theMEMS parts, it must be understood that other materials can be used.Other applications can use materials to hold the rollingspheres/bearings that do not fully dissipates during the thermal cyclebut simply liquefy to a point of not being of impediments on themovement of the spheres while the joining of the moving parts isachieved and then resolidify when the parts cool down. These materialscomprise natural or synthetic waxes or rosins e.g., paraffin orcolophony (pine tree resin). Likewise, the thermally dissipativematerial compositions can include the usage of hydrocolloids, e.g. gumarabic, gum karaya, gum agar, guam gum.

Naturally, in order to satisfy local and specific requirements, a personskilled in the art may apply to the solution described above manymodifications and alterations all of which, however, are included withinthe scope of protection of the invention as defined by the followingclaims.

The invention claimed is:
 1. A method for improving alignment precisionof two parts of a Micro-Electromechanical Systems (MEMS), said methodcomprising the steps of: forming a vertical stack of said two partswhereby said two parts are separated by at least one rolling elementhaving a first diameter; pivoting one of said two parts about said atleast one rolling element to bring said two parts into horizontalalignment; locking said at least two parts together whereby said atleast two parts remain spaced by said first diameter, said lockingachieved by solidification of solder; and prior to said forming step,maintaining said at least one rolling element on one of said two partsusing a thermally dissipative material that comprises glycerol.
 2. Themethod of claim 1 further comprising, wherein a magnetic field imposesz-direction forces to accomplish said pivoting step.
 3. The method ofclaim 1 further comprising the step of freeing said at least one rollingelement before aligning said two parts.
 4. The method of claim 1 whereinsaid rolling element is either a sphere or a bearing.
 5. The method ofclaim 1 wherein at least one of said two parts comprises a cavity, saidrolling element being centered in said cavity before aligning said twoparts.
 6. The method according to claim 1 wherein the upper one of saidtwo parts comprises a conical cavity adapted to cooperate with saidrolling element.
 7. The method of claim 1 wherein said rolling elementcomprises tungsten carbide or steel.